Despite the great amount of engineering attention given in the past to the problem of single sheet feeding, there is a continuing need for improved machines, to overcome limitations to error free, high speed sheet feeding.
In particular, improvements are needed in singulating sheets when they are provided to a machine as a stack, and for delivering sheets to singulators in ways which enhance, rather than sometimes make more difficult, their function. Often, it is necessary to have first and second singulators to avoid the tendency for double feeding of sheets.
Better machines are needed for reliably singulating sheets and other articles which vary in thickness or surface character. Articles which cause feeding problems or tend to be damaged include articles having step change in thickness, such as envelopes with flaps fed first, articles with body joints, window envelopes, plastic credit cards affixed to paper sheets, and heavily coated or printed articles, etc.
Most prior art sheet feeding machines have a large number of components. This is disadvantageous, not only because of increased manufacturing cost, but because complexity and multiple degrees of freedom make it difficult to obtain and maintain precision adjustment. Feeding sheets from the top of a stack in prior art machines has typically required complex sets of rollers and adjusting mechanisms. Typically, the elevation of the top of the stack must be maintained, as by continuously raising the stack. Alternatively, the stack is inclined on edge to make the sheets partially shingled. There is a need for simplification.
Retarders for singulators also need improvement. When they are soft they tend to wear; when they are abrasive, they tend to scrape the article surface. Debris accretes on retarders, especially toner from printed sheets, causing them to lose effectiveness. Still another deficiency in the prior art is that, when sheets are taken away from a singulator at increased speed, there is a tendency for slipping and rubbing on the surface of articles at either the take-away rollers or singulator.